Accompanied with recent electronic equipments to be “light weight”, “thin” and “downsizing”, a thickness of a laminate used for a thin package and thin substrate such as circuit substrates including, for example, memory cards, portable telephones and the like which require space saving needs to be 0.5 mm or less, and preferably 0.2 mm or less. As the thickness of a laminate gets thinner and thinner, there occur more problems in that warpage or bending easily occurs in the process such as a circuit formation and the like or in the general course of carrying. These problems are caused by the fact that the rigidity of a substrate, in this case, the rigidity of a substrate at room temperature, is not sufficient as a substrate and laminates become thin.
Meanwhile, when a thin substrate was used as described above, and even when the thin substrate was exposed at high temperature in a semiconductor mounting process using ACP, ACF and the like requiring a high temperature or in a second mounting process using reflow soldering and the like, there have been problems of the mounting defect and the like such that the substrate warpage or the substrate bending occur, due to the result of the stress caused by the different thermal expansion coefficients. This resulted from the fact that the rigidity of a laminate at high temperature was not sufficient, relative to the stress caused by the high temperature.
In order to solve the above problems such as warpage during carrying while in a process, defect upon mounting and the like, it has been required for substrate materials to have high rigidity both at the room temperature and at high temperatures.
To achieve high rigidity, there have been proposed general methods including a method for making a glass fiber cloth in use to a glass having high hardness (S glass, H glass and the like), a method for increasing the folding number of the glass fiber cloth in use, and the like (refer to JP2001-329080A). However, there were drawbacks in that the glasses having high hardness were expensive as compared to glass fiber cloths in general (E glass), and that the workability including drilling and the like was inferior. Meanwhile, when the folding number was increased, there were problems in that the number of prepregs to use was increased per one sheet of a laminate so that the cost became higher, or the like.
On the other hand, there has also been reviewed such a method for increasing the rigidity by combining an inorganic filler into a resin composition. In particular, there has been known a method for increasing the rigidity of a resin composition by comprising high volume fraction of inorganic filler (refer to JP96-216335A). However, as the volume fraction of the inorganic filler was increased, there might easily occur problems in that detachment takes place at the surface of a resin and filler during processing, and the adhesive strength between prepregs in the laminate was deteriorated for easily causing peeling off.
Furthermore, when an inorganic filler having high rigidity such as silica was used, there have occurred problems in that abrasion of drill was remarkable during drill processing, hole positioning accuracy was deteriorated, hole wall coarseness was getting worse, and the like. When an inorganic filler having high rigidity such as silica or the like was used, a filler having a specific diameter was mixed (refer to JP2003-020407A) or a filler having low hardness was used together (refer to JP2000-117733A) in order to solve the problems. However, the effect was not sufficient.